U.S. patent number 8,394,155 [Application Number 12/266,770] was granted by the patent office on 2013-03-12 for thermally stabilized bag house filters and media.
The grantee listed for this patent is Young H Kim, Anil Kohli, Antoine Schelling, Hageun Suh, B. Lynne Wiseman, Kurt Hans Wyss. Invention is credited to Young H Kim, Anil Kohli, Antoine Schelling, Hageun Suh, B. Lynne Wiseman, Kurt Hans Wyss.
United States Patent |
8,394,155 |
Kohli , et al. |
March 12, 2013 |
Thermally stabilized bag house filters and media
Abstract
A bag filter having a support structure clothed in a filter bag.
The cloth of the filter bag is a composite of at least one
substrate layer and at least one nanoweb bonded thereto in a
face-to-face relationship. The nanoweb is positioned at the surface
of the filter bag first exposed to the hot particle laden gas
stream and can have a basis weight of greater than about 0.1
gsm.
Inventors: |
Kohli; Anil (Midlothian,
VA), Schelling; Antoine (Geneva, CH), Wyss; Kurt
Hans (Chavannes de Bogis, CH), Wiseman; B. Lynne
(Richmond, VA), Kim; Young H (Hockessin, DE), Suh;
Hageun (Chadds Ford, PA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kohli; Anil
Schelling; Antoine
Wyss; Kurt Hans
Wiseman; B. Lynne
Kim; Young H
Suh; Hageun |
Midlothian
Geneva
Chavannes de Bogis
Richmond
Hockessin
Chadds Ford |
VA
N/A
N/A
VA
DE
PA |
US
CH
CH
US
US
US |
|
|
Family
ID: |
40351867 |
Appl.
No.: |
12/266,770 |
Filed: |
November 7, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090255226 A1 |
Oct 15, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61002605 |
Nov 9, 2007 |
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Current U.S.
Class: |
55/382; 55/DIG.2;
55/486; 55/524; 15/347 |
Current CPC
Class: |
B01D
46/02 (20130101); B01D 39/1623 (20130101); B01D
46/546 (20130101); B01D 2239/025 (20130101); B01D
2275/10 (20130101) |
Current International
Class: |
B01D
46/02 (20060101) |
Field of
Search: |
;55/486,487,382,528,DIG.2,DIG.5 ;15/347 ;96/66,68 ;264/258,DIG.48
;442/389 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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03/080905 |
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Oct 2003 |
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WO |
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2006107847 |
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Oct 2006 |
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WO |
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Other References
PCT International Search Report and Written Opinion for
International Application No. PCT/US2008/082770 dated Nov. 7, 2008.
cited by applicant .
Kohli et al., U.S. Appl. No. 61/002,605, filed Nov. 9, 2007. cited
by applicant .
Skirius et al., U.S. Appl. No. 60/950,269, filed Jul. 17, 2007.
cited by applicant .
Gross et al., U.S. Appl. No. 60/947,266, filed Jun. 29, 2007. cited
by applicant .
Skirius et al., U.S. Appl. No. 60/880,873, filed Jan. 16, 2007.
cited by applicant .
Gross et al., U.S. Appl. No. 60/848,105, filed Sep. 29, 2006. cited
by applicant .
Gross et al., U.S. Appl. No. 60/817,749, filed Jun. 30, 2006. cited
by applicant .
Skirius et al., U.S. Appl. No. 60/760,323, filed Jan. 18, 2006.
cited by applicant .
Gross et al., U.S. Appl. No. 60/729,264, filed Oct. 21, 2005. cited
by applicant .
Boehmer et al., U.S. Appl. No. 60/667,873, filed Apr. 1, 2005.
cited by applicant .
ES Fibervisions, AL-Adhesion-C--Improved Airlaid Fiber, Apr. 2010,
MD.41.40.00-ver05. cited by applicant .
Units of Textile Measurement, http.//en.wikipedia.org/wiki, website
visited Sep. 10, 2012. cited by applicant.
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Primary Examiner: Smith; Duane
Assistant Examiner: Pham; Minh-Chau
Attorney, Agent or Firm: Andrus, Sceales, Starke &
Sawall, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority under 35 U.S.C. .sctn.119 from
U.S. Provisional Application Ser. No. 61/002,605 (filed Nov. 9,
2007), the disclosure of which is incorporated by reference herein
for all purposes as if fully set forth.
Claims
We claim:
1. A bag filter comprising a support structure clothed in a filter
bag, the cloth of said filter bag comprising a composite of at
least one substrate layer and a first nanoweb layer having a basis
weight of greater than about 0.1 gsm bonded thereto in a
face-to-face relationship, wherein the nanoweb comprises polyamide
nanofibers incorporating an effective amount of an antioxidant.
2. The bag filter of claim 1 in which the antioxidant is selected
from the group consisting of a hindered phenol, a copper halide, a
phenolic amide, a phenolic ester, an organic salt of copper, a
potassium iodide and stearate mixture, a copper acetate and
potassium bromide mixture, a hindered amine, a polymeric hindered
phenol, a hindered phosphite, and combinations or blends
thereof.
3. The bag filter of claim 2, wherein the antioxidant is a hindered
phenol.
4. The bag filter of claim 3, wherein the hindered phenol is
N,N'-hexamethylene
bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide).
5. The bag filter of claim 1, wherein the nanoweb is positioned on
the upstream side of the filter bag.
6. The bag filter of claim 1, wherein the substrate layer and
nanoweb layer remain bonded after the filter has been subjected to
VDI 3926 for 30 cycles.
7. The bag filter of claim 1, further comprising a second nanoweb
layer bonded to the substrate layer on the face opposite to the
face bonded to the first nanoweb layer.
8. The bag filter of claim 7, further comprising a second substrate
layer bonded to the second nanoweb layer and located on the
upstream side of the filter bag.
9. The bag filter of claim 1, wherein at least one nanoweb layer
and at least one substrate layer are bonded by a method selected
from the group consisting of ultrasonic bonding, thermal bonding,
adhesive bonding, needlepunching and hydroentangling.
10. The bag filter of claim 7, wherein the substrate layer and the
nanoweb layer are needle punched with about 40 to 100
perforations/cm.sup.2, and 25% or less of the nanoweb layer is
perforated.
11. The bag filter of claim 1, wherein each substrate layer
independently comprises fiber selected from polyester fiber, carbon
fiber, polyimide fiber, glass fiber, and mixtures thereof.
12. A bag filter comprising a support structure clothed in a filter
bag, the cloth of said filter bag comprising a composite of a first
substrate layer bonded in a face to face relationship to a nanoweb
layer having a basis weight of greater than about 0.1 gsm and a
second substrate layer bonded to the nanoweb layer and wherein the
nanoweb is positioned on the upstream side of the filter bag,
wherein the nanoweb comprises polyamide nanofibers incorporating an
effective amount of an antioxidant.
13. The bag filter of claim 12, wherein the second substrate layer
is positioned in between the nanoweb and the first substrate
layer.
14. The bag filter of claim 12, wherein the nanoweb layer is
positioned between the first substrate layer and the second
substrate layer.
15. The bag filter of claim 12, wherein the substrate layer and
nanoweb layer remain bonded after the filter has been subjected to
VDI 3926 for 30 cycles.
Description
FIELD OF THE INVENTION
This invention relates filters and to composites useful as filters
in filtration of solids from fluid streams, as, for example, in
industrial gas streams.
BACKGROUND
Dust collectors, also known as bag houses, are generally used to
filter particulate material from industrial effluent or off-gas.
Once filtered, the cleaned off-gas can be vented to the atmosphere
or recycled. Such a bag house dust collector structure generally
includes one or more flexible filter banks supported within a
cabinet or similar structure. In such a filter cabinet and bank,
the filter bag is generally secured within the cabinet and
maintained in a position such that effluent efficiently passes
through the bag thereby removing entrained particulates. The filter
bag, secured within the cabinet, is typically supported by a
structure that separates the upstream and downstream air and
supports the filter bag to maintain efficient operation.
More specifically, in a so-called "baghouse filter", particulate
material is removed from a gaseous stream as the stream is directed
through the filter media. In a typical application, the filter
media has a generally sleeve-like tubular configuration, with gas
flow arranged so as to deposit the particles being filtered on the
exterior of the sleeve. In this type of application, the filter
media is periodically cleaned by subjecting the media to a pulsed
reverse-flow, which acts to dislodge the filtered particulate
material from the exterior of the sleeve for collection in the
lower portion of the baghouse filter structure. U.S. Pat. No.
4,983,434 illustrates a baghouse filter structure and a prior art
filter laminate.
The separation of particulate impurities from industrial fluid
streams is often accomplished using fabric filters. These textile
based filter media remove particulate from the fluids. When the
resistance to flow or pressure drop through the textile caused by
accumulation of particulate on the filter becomes significant, the
filter must be cleaned, and the particulate cake removed.
It is common in the industrial filtration market to characterize
the type of filter bag by the method of cleaning. The most common
types of cleaning techniques are reverse air, shaker and pulse jet.
Reverse air and shaker techniques are considered low energy
cleaning techniques.
The reverse air technique is a gentle backwash of air on a filter
bag which collects dust on the interior. The back wash collapses
the bag and fractures dust cake which exits the bottom of the bag
to a hopper.
Shaker mechanisms clean filter cake that collects on the inside of
a bag as well. The top of the bag is attached to an oscillating arm
which creates a sinusoidal wave in the bag to dislodge the dust
cake.
Pulse jet cleaning techniques employs a short pulse of compressed
air that enters the interior top portion of the filter tube. As the
pulse cleaning air passes through the tube venturi it aspirates
secondary air and the resulting air mass violently expands the bag
and casts off the collected dust cake. The bag will typically snap
right back to the cage support and go right back into service
collecting particulate.
Of the three cleaning techniques the pulse jet is the most
stressful on the filter media. However, in recent years industrial
process engineers have increasingly selected pulse jet
baghouses.
The need for high temperature (up to 200.degree. C.), thermally
stable, chemically resistant filter media in baghouses narrows the
choice of filter media to only a few viable candidates for pulse
jet applications. Common high temperature textiles comprise
polytetrafluoroethylene (PTFE), fiberglass, or polyimides
(polyimides are stable for continuous use to 260.degree. C.). When
the effect of high temperature is combined with the effect of
oxidizing agents, acids or bases, there is a tendency for
fiberglass and polyimide media to fail prematurely. Thus, there is
a preference for using PTFE. Commercially available PTFE fabrics
are supported needlefelts of PTFE fiber. These felts usually weight
from 20-26 oz/yd.sup.2 and are reinforced with a multifilament
woven scrim (4-6 oz/yd.sup.2). The felts are made up of staple
fibers, (usually 6.7 denier/filament, or 7.4 dtex/filament) and 2-6
inches in length. This product works similarly to many other felted
media in that a primary dust cake "seasons" the bag. This
seasoning, sometimes called in-depth filtration, causes the media
to filter more efficiently but has a drawback in that the pressure
drop increases across the media during use. Eventually the bag will
blind or clog and the bags will have to be washed or replaced. In
general, the media suffers from low filtration efficiency, blinding
and dimensional instability (shrinkage) at high temperatures.
Another type of structure designed for high temperatures is
described in U.S. Pat. No. 5,171,339. A bag filter is disclosed
that comprises a bag retainer clothed in a filter bag. The cloth of
said filter bag comprises a laminate of a felt of poly(m-phenylene
isophthalamide), polyester or polyphenylenesulfide fibers having a
thin nonwoven fabric of poly(p-phenylene terephthalamide) fibers
needled thereto, the poly(p-phenylene terephthalamide) fabric being
positioned at the surface of the filter bag first exposed to the
hot particle laden gas stream. The poly(p-phenylene
terephthalamide) fabric can have a basis weight of from 1 to 2
oz/yd.sup.2.
A two layer product of porous expanded PTFE membrane (ePTFE)
laminated to woven porous expanded PTFE fiber fabric has also been
used. Commercial success of this product has not been realized due
to several reasons, but primarily due to the woven fiber fabric
backing not wearing well on the pulse jet cage supports. The woven
yarns slide on themselves and create excessive stress on the
membrane, resulting in membrane cracks.
Nonwoven fabrics have been advantageously employed for manufacture
of filter media. Generally, nonwoven fabrics employed for this type
of application have been entangled and integrated by mechanical
needle-punching, sometimes referred to as "needle-felting", which
entails repeated insertion and withdrawal of barbed needles through
a fibrous web structure. While this type of processing acts to
integrate the fibrous structure and lend integrity thereto, the
barbed needles inevitably shear large numbers of the constituent
fibers, and undesirably create perforations in the fibrous
structure, which act to compromise the integrity of the filter and
can inhibit efficient filtration. Needle-punching can also be
detrimental to the strength of the resultant fabric, requiring that
a suitable nonwoven fabric have a higher basis weight in order to
exhibit sufficient strength for filtration applications.
U.S. Pat. No. 4,556,601 to Kirayoglu discloses a hydroentangled,
nonwoven fabric, which may be used as a heavy-duty gas filter. This
filtration material however, cannot be subjected to a shrinkage
operation. Exposure of the described fabric to a shrinkage
operation is believed to have a negative effect on the physical
performance of the filtration material.
U.S. Pat. No. 6,740,142 discloses nanofibers for use in baghouse
filters. A flexible bag is at least partially covered by a layer
having a basis weight of 0.005 to 2.0 grams per square meter (gsm)
and a thickness of 0.1 to 3 microns. The layer comprises a
polymeric fine fiber with a diameter of about 0.01 to about 0.5
micron, but is limited in basis weight due to the limitations of
the process used to produce it. The limitation in basis weight of
the layer in the '142 patent significantly reduces the lifetime of
the filter medium and severely reduces the ability of the filter to
survive cleaning cycles.
SUMMARY OF THE INVENTION
A first embodiment of the present invention is a bag filter
comprising a support structure clothed in a filter bag, the cloth
of said filter bag comprising a composite of at least one substrate
layer and a first nanoweb layer having a basis weight of greater
than about 0.1 gsm bonded thereto in a face-to-face relationship.
The nanoweb comprises nanofibers spun from a polyamide
incorporating an effective amount of an antioxidant.
Another embodiment of the present invention is a bag filter
comprising a support structure clothed in a filter bag, the cloth
of said filter bag comprising a composite of a first substrate
layer bonded in a face to face relationship to a nanoweb layer
having a basis weight of greater than about 0.1 gsm and a second
substrate layer bonded to the nanoweb layer, wherein the nanoweb is
positioned on the upstream side of the filter bag. The nanoweb
comprises polyamide nanofibers incorporating an effective amount of
an antioxidant.
DETAILED DESCRIPTION
The present invention is directed to a filter media which is formed
through bonding of a nanoweb layer to a substrate by
hydroentanglement, needle punching, or other bonding means. This
construction provides a filter media having the requisite strength
characteristics, without possessing the limited performance of the
product of U.S. Pat. No. 6,740,142. The filtration media of the
present invention also demonstrates a highly desirable uniformity
for cost-effective use and stability over long periods of use.
The term "nanofiber" as used herein refers to fibers having a
number average diameter or cross-section less than about 1000 nm,
even less than about 800 nm, even between about 50 nm and 500 nm,
and even between about 100 and 400 nm. The term diameter as used
herein includes the greatest cross-section of non-round shapes.
The term "nonwoven" means a web including a multitude of randomly
distributed fibers. The fibers generally can be bonded to each
other or can be unbonded. The fibers can be staple fibers or
continuous fibers. The fibers can comprise a single material or a
multitude of materials, either as a combination of different fibers
or as a combination of similar fibers each comprised of different
materials. A "nanoweb" is a nonwoven web that comprises
nanofibers.
A "substrate" is a support layer and can be any planar structure to
which the nanoweb layer can be bonded, adhered or laminated.
Advantageously, the substrate layers useful in the present
invention are spunbond nonwoven layers, but can be made from carded
webs of nonwoven fibers and the like.
By "effective amount" of antioxidant is meant an amount that
provides the desired level of thermal stability to the filter as
measured by physical or visual properties.
The object of the present invention is to provide a thermally
stable, high-efficiency dust-collecting filter cloth for bag filter
units for exhaust gas dust collection, and to provide a bag filter
comprising the filter cloth. The filter includes at least one
nanoweb layer in combination with at least one substrate layer in a
mechanically stable filter structure. These layers together provide
excellent filtering and high particle capture efficiency at minimum
fluid flow restriction through the filter media. The substrate can
be positioned in the fluid stream upstream, downstream or in an
internal layer.
In one embodiment the filter comprises a filtration medium
including a thermally-stabilized nanoweb layer having a basis
weight of greater than about 0.1 gsm, or greater than about 0.5
gsm, or greater than about 5 gsm, or even greater than about 10 gsm
and up to about 90 gsm. The filtration medium further comprises a
substrate to which the nanoweb is bonded in a face-to-face
relationship. Advantageously, the nanoweb layer is positioned on
the upstream surface or side of the filter bag, i.e. on the surface
which is first exposed to the hot, particle-laden gas stream.
In a further embodiment the filter comprises a composite of a first
substrate layer having a thermally-stabilized nanoweb bonded
thereto in a face-to-face relationship, the nanoweb being
positioned on the upstream side of the filter bag, i.e. at the
surface of the filter bag first exposed to the hot, particle-laden
gas stream, wherein the nanoweb has a basis weight of greater than
about 0.1 gsm, and a second substrate layer bonded to the nanoweb
layer. In some cases it is advantageous that the second substrate
layer is positioned in between the nanoweb and the first substrate
layer, while in other cases it is desirable that the nanoweb layer
be positioned between the first and second substrate layers.
Polymers useful for electroblowing nanofiber webs of the present
invention are polyamides (PA), and preferably a polyamide selected
from the group consisting of polyamide 6, polyamide 6,6, polyamide
6,12, polyamide 11, polyamide 12, polyamide 4,6, a semi-aromatic
polyamide (high temperature polyamide) and any combination or blend
thereof. The polyamides (PA) used in preparing the blending
composition of the invention are well known in the art.
Representative polyamides include semicrystalline and amorphous
polyamide resins of a molecular weight of at least 5,000 as
described, for instance, in U.S. Pat. Nos. 4,410,661; 4,478,978;
4,554,320; and 4,174,358.
In accordance with the invention, polyamides obtained by
copolymerization of two of the above polymers, by terpolymerization
of the above polymers or their component monomers, e.g., a
copolymer of adipic acid, isophthalic acid and
hexamethylenediamine, or blended mixtures of polyamides such as a
mixture of PA 6, 6 and PA 6 may also be used. Preferably, the
polyamides are linear and have melting points or softening points
above 200.degree. C.
The polyamide used to spin the fibers comprises a thermal stability
additive, such as an antioxidant. Suitable antioxidants for use in
the invention are any materials that are soluble in the spinning
solvent with the polyamide if the polyamide is spun from solution.
Examples of such materials are copper halides and hindered phenols.
By "hindered phenol" is meant a compound whose molecular structure
contains a phenolic ring in which one or both of the carbon atoms
cis to the hydroxyl moiety holds an alkyl group. The alkyl group is
preferably a tertiary butyl moiety and both adjacent carbon atoms
hold a tertiary butyl moiety.
Antioxidants that are useful for this invention include: phenolic
amides such as N,N'-hexamethylene
bis(3,5-di-(tert)-butyl-4-hydroxyhydrocinnamamide) (Irganox 1098);
amines such as various modified benzenamines (e.g. Irganox 5057);
phenolic esters such as
ethylenebis(oxyethylene)bis-(3-(5-tert-butyl-4-hydroxy-m-tolyl)-propionat-
e (Irganox 245) (all available from Ciba Specialty Chemicals Corp.,
Tarrytown, N.Y.); organic or inorganic salts such as mixtures of
cuprous iodide, potassium iodide, and zinc salt of octadecanoic
acid, available as Polyad 201 (from Ciba Specialty Chemicals Corp.,
Tarrytown, N.Y.), and mixtures of cupric acetate, potassium
bromide, and calcium salt of octadecanoic acid, available as Polyad
1932-41 (from Polyad Services Inc., Earth City, Mo.); hindered
amines such as
1,3,5-triazine-2,4,6-triamine,N,N'''-[1,2-ethane-diyl-bis[[[4,6-bis-[buty-
l
(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3-
,1-propanediyl]]bis[N',N''-dibutyl-N',N''-bis(1,2,2,6,6-pentamethyl-4-pipe-
ridinyl) (Chimassorb 119 FL), 1,6-hexanediamine,
N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)-polymer with
2,4,6-trichloro-1,3,5-triazine, reaction products with
N-butyl-1-butanamine an
N-butyl-2,2,6,6-tetramethyl-4-piperidinamine (Chimassorb 2020), and
poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][2,2,6,-
6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-p-
iperidinyl)imino]]) (Chimassorb 944) (all available from Ciba
Specialty Chemicals Corp., Tarrytown, N.Y.); polymeric hindered
phenols such as 2,2,4 trimethyl-1,2 dihydroxyquinoline (Ultranox
254 from Crompton Corporation, a subsidiary of Chemtura
Corporation, Middlebury, Conn., 06749); hindered phosphites such as
bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite (Ultranox 626
from Crompton Corporation, a subsidiary of Chemtura Corporation,
Middlebury, Conn., 06749); and tris(2,4-di-tert-butyl-phenyl)
phosphite (Irgafos 168 from Ciba Specialty Chemicals Corp.,
Tarrytown, N.Y.); 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic
acid (Fiberstab PA6, available from Ciba Specialty Chemicals Corp.,
Tarrytown, N.Y.), and combinations and blends thereof.
To achieve the desired improvement in filter performance, the
concentration of the antioxidant agent used as stabilizer in the
method of the invention is preferably between 0.01 and 10% by
weight relative to the polyamide and especially preferably between
0.05 and 5% by weight. Especially good results are achieved if the
concentration of antioxidant agent lies between 0.2 and 2.5% by
weight relative to the polyamide used.
The filter of the invention can be used in a variety of filtration
applications including pulse clean and non-pulse cleaned filters
for dust collection, gas turbines and engine air intake or
induction systems, gas turbine intake or induction systems, heavy
duty engine intake or induction systems, light vehicle engine
intake or induction systems, Zee filter, vehicle cabin air, off
road vehicle cabin air, disk drive air, photocopier-toner removal,
HVAC filters for both commercial or residential filtration
applications, and vacuum cleaner applications.
The substrate layers of the invention can be formed from a variety
of conventional fibers including cellulosic fibers such as cotton,
hemp or other natural fibers, inorganic fibers including glass
fibers, carbon fibers or organic fibers such as polyesters,
polyimides, polyamides, polyolefins, or other conventional fibers
or polymeric materials and mixtures thereof.
The substrate layers of the filter bag of the invention can be
woven or non-woven. In woven bags, the fibers are typically formed
into an interlocking mesh of fiber in a typical woven format.
Non-woven fabrics are typically made by loosely forming the fibers
in no particular orientation and then binding the fibers into a
filter fabric. One preferred mode of constructing the elements of
the invention includes using a felt media as a substrate. Felts are
a compressed, porous, non-woven fabric made by laying discrete
natural or synthetic fibers and compressing the fibers into a felt
layer using commonly available felt bonding technology that would
be known to one skilled in the art.
Fibers are typically used which result in fabrics that exhibit
excellent resilience and resistance to the effects of the passage
of air and the entrapment of particulates. The fabrics can have
stability with respect to chemical particulates, and can be stable
with respect to varying temperatures of both the air passing
through the bag house and the temperature of the particulate
entrained on the filter surface.
The filter structures of the invention are typically maintained in
their useful open shape by supporting the substrate plus nanoweb
layer composite on a suitable support structure such as a retainer
at the neck of a bag, or a support structure can be located in the
interior of the bag. Such supports can be formed from linear
members in the form of a wound wire or cage-like structure.
Alternatively, the support can comprise a perforated ceramic or
metal structure that mimics the shape of the bag. If the support
structure contacts the filter substrate over a significant fraction
of its surface area, the support structure should be permeable to
the passage of air through the structure and should provide no
incremental increase in pressure drop over the filter bag. Such
support structures can be formed such that they contact the
entirety of the interior of the filter bag and maintain the filter
bag in an efficient filtration shape or confirmation.
A process for combining the nanoweb layers with the substrate to
produce the present composite structure is not specifically
limited. The nanofibers of the nanoweb layer can be physically
entwined in the substrate layer, or they can be bonded by
inter-fusion of the fibers of the nanoweb layer with those of the
substrate, for example by thermal, adhesive or ultrasonic
lamination or bonding.
Thermal methods for bonding the substrate layer to the nanoweb
layer or a nanoweb plus substrate layer include calendering.
"Calendering" is the process of passing a web through a nip between
two rolls. The rolls may be in contact with each other, or there
may be a fixed or variable gap between the roll surfaces.
Advantageously, in the calendering process, the nip is formed
between a soft roll and a hard roll. The "soft roll" is a roll that
deforms under the pressure applied to keep two rolls in a calender
together. The "hard roll" is a roll with a surface in which no
deformation that has a significant effect on the process or product
occurs under the pressure of the process. An "unpatterned" roll is
one which has a smooth surface within the capability of the process
used to manufacture them. There are no points or patterns to
deliberately produce a pattern on the web as it passed through the
nip, unlike a point bonding roll. The hard roll in the process of
calendering used in the present invention can be patterned or
unpatterned.
Adhesive lamination can be carried out in conjunction with
calendering or by application of pressure by other means to the
laminate in the presence of a solvent based adhesive at low
temperatures, for example room temperature. Alternatively a hot
melt adhesive can use used at elevated temperatures. One skilled in
the art will readily recognize suitable adhesives that can be used
in the process of the invention.
Examples of methods of entwining the fibers according to such a
physical bonding are needle punch processing and water-jet
processing, otherwise known as hydroentangling or spun lacing.
Needle punching (or needling) consists essentially of tucking a
small bundle of individual fibers down through a carded batt of
fibers in such large numbers of penetrations that a cohesive
textile structure is formed, as disclosed in U.S. Pat. Nos.
3,431,611 and 4,955,116
For the process of manufacturing the filter of the present
invention it is desirable to perform needle punch processing (or
water-jet processing) on the high-density layer (substrate) side of
the nonwoven fabric. Compared to the case where needle punch
processing is performed on the low-density layer (nanoweb) side,
needle punch processing on the high-density layer side can suppress
collapse or deformation of the pores accompanied by intertwining,
as well as undesirable widening of the pore size, thereby
suppressing lowering of the initial cleaning efficiency with
respect to smaller particles. It is preferable to set the number of
needles (the number for penetration) per unit area in the range
from about 40 to about 100 perforations/cm.sup.2, in order to
suppress undesirable widening of the pore diameter, and to perform
sufficient intertwining operation. Further, no more than about 25%
of the surface area of the low density layer should be
perforated.
The as-spun nanoweb comprises primarily or exclusively nanofibers,
advantageously produced by electrospinning, such as classical
electrospinning or electroblowing, and in certain circumstances, by
meltblowing or other such suitable processes. Classical
electrospinning is a technique illustrated in U.S. Pat. No.
4,127,706, wherein a high voltage is applied to a polymer in
solution to create nanofibers and nonwoven mats. However, total
throughput in electrospinning processes is too low to be
commercially viable in forming heavier basis weight nanowebs.
The "electroblowing" process is disclosed in World Patent
Publication No. WO 03/080905. A stream of polymeric solution
comprising a polymer and a solvent is fed from a storage tank to a
series of spinning nozzles within a spinneret, to which a high
voltage is applied and through which the polymeric solution is
discharged. Meanwhile, compressed air that is optionally heated is
issued from air nozzles disposed in the sides of, or at the
periphery of the spinning nozzle. The air is directed generally
downward as a blowing gas stream which envelopes and forwards the
newly issued polymeric solution and aids in the formation of the
fibrous web, which is collected on a grounded porous collection
belt above a vacuum chamber. The electroblowing process permits
formation of commercial sizes and quantities of nanowebs at basis
weights in excess of about 1 gsm, even as high as about 40 gsm or
greater, in a relatively short time period.
A substrate can be arranged on the collector so as to collect and
combine the nanofiber web spun on the substrate. Examples of the
substrate may include various nonwoven cloths, such as meltblown
nonwoven cloth, needle-punched or spunlaced nonwoven cloth, woven
cloth, knitted cloth, paper, and the like, and can be used without
limitations so long as a nanofiber layer can be added on the
substrate. The nonwoven cloth can comprise spunbond fibers,
dry-laid or wet-laid fibers, cellulose fibers, melt blown fibers,
glass fibers, or blends thereof. Alternatively, the nanoweb layer
can be deposited directly onto the felt substrate.
It can be advantageous to add known-in-the-art plasticizers to the
various polymers described above, in order to reduce the T.sub.g of
the fiber polymer. Suitable plasticizers will depend upon the
polymer to be electrospun or electroblown, as well as upon the
particular end use into which the nanoweb will be introduced. For
example, nylon polymers can be plasticized with water or even
residual solvent remaining from the electrospinning or
electroblowing process. Other known-in-the-art plasticizers which
can be useful in lowering polymer T.sub.g include, but are not
limited to aliphatic glycols, aromatic sulphanomides, phthalate
esters, including but not limited to those selected from the group
consisting of dibutyl phthalate, dihexl phthalate, dicyclohexyl
phthalate, dioctyl phthalate, diisodecyl phthalate, diundecyl
phthalate, didodecanyl phthalate, and diphenyl phthalate, and the
like. The Handbook of Plasticizers, edited by George Wypych, 2004
Chemtec Publishing, incorporated herein by reference, discloses
other polymer/plasticizer combinations which can be used in the
present invention.
TEST METHODS
In the non-limiting examples that follow, the following test
methods were employed to determine various reported characteristics
and properties. ASTM refers to the American Society of Testing
Materials. ISO refers to the International Standards Organization.
TAPPI refers to Technical Association of Pulp and Paper
Industry.
Filtration Efficiency, Pressure Drop and Cycle Time were measured
according VDI 3926, the text of which is incorporated herein by
reference.
According to VDI 3926, the filtration efficiency (also called dust
leakage) is measured in micrograms per cubic meter, pressure drop
in Pascal (Pa) and cycle time is measured in seconds. The
filtration efficiency represents the amount of dust passing through
the filter. The pressure drop is the differential pressure between
the 2 faces of the filters. The cycle time is the duration between
2 pulses to release the dust cake. When a certain pressure drop is
obtained (in VDI 3926 the maximum pressure drop is set at 1000 Pa)
a reverse flow pulse is automatically created. The VDI 3926
procedure is based on an initial 30 cycles, followed by 10,000
cycles to simulate filter aging, and finally another 30 cycles. The
filtration efficiency, pressure drop and cycle time are measured at
the end of the final cycles.
Air Permeability is measured according to ISO 9237, and is reported
in units of I/dm.sup.2/min. The basis wt was measured according to
ISO 3801
Basis Weight of the web was determined by ASTM D-3776, which is
hereby incorporated by reference and reported in g/m.sup.2.
Fiber Diameter was determined as follows. Ten scanning electron
microscope (SEM) images at 5,000.times. magnification were taken of
each nanofiber layer sample. The diameter of eleven (11) clearly
distinguishable nanofibers were measured from the photographs and
recorded. Defects were not included (i.e., lumps of nanofibers,
polymer drops, intersections of nanofibers). The average (mean)
fiber diameter for each sample was calculated.
Tensile Strength was measured according to ASTM D5035-95, "Standard
Test Method for Breaking Force and Elongation of Textile Fabrics
(Strip Method)" and was reported in kg/cm.sup.2.
EXAMPLES 1-5
In order to test the thermal stability of the nanowebs, nanowebs
were produced with a basis weight of 15 grams per square meter
(gsm) spun from polyamide PA 6/6 (Zytel 3218, DuPont, Wilmington,
Del.) nanofiber using the process of World Patent Publication No.
WO 03/080905. Mean fiber diameter was about 400 nm. Hand samples
(20 cm.times.25 cm) were suspended in a forced circulation oven at
140.degree. C. A sample was removed daily and examined for color
and shrinkage, and tested for tensile strength and elongation.
Table 1 summarizes the samples and levels of antioxidant used, and
color after 21 days of aging at 140.degree. C. The antioxidant used
was N,N'-hexamethylene
bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide (Irganox 1098,
Ciba, Tarrytown, N.Y.) or Copper Bromide (PolyAD 1932,
Supplier).
Shrinkage was measured by measuring the length of one side of the
sample and expressing it as a percentage of the original sample
length.
TABLE-US-00001 TABLE 1 Shrinkage AO level % Color after after 21
Sample AO Type of polymer 21 days days (%) 1 Irganox 1098 0 Dark
Yellow 88 2 Irganox 1098 0.75 Slight Yellow 96.5 3 Irganox 1098 1.0
Very Slight Yellow 102 4 Irganox 1098 2.0 Very Slight Yellow 97 5
PolyAD 1932 0.2 Slight Yellow 97.5
The samples containing antioxidant demonstrated greatly improved
resistance to color change on aging, which is believed to represent
greatly improved thermal stability over the control sample (sample
1), which contained no antioxidant.
Table 2 shows tensile strength retention as a percentage of initial
tensile strength and absolute elongation after aging for 21 days.
The initial elongation to break of the unaged samples was averaged
to be 23%.
TABLE-US-00002 TABLE 2 Sample Strength (%) Elongation (%) 1 16 4 2
88 19 3 88 16 4 94 19 5 80 16
The samples containing antioxidant showed marked improvements in
retention of tensile strength after aging at high temperature, as
compared to control sample 1.
Nanoweb samples were also laminated to felts in order to test the
thermal stability of the laminates, as described in examples 6-10
below.
EXAMPLES 6-10
Five different nanowebs were produced with a basis wt of .about.10
gsm and with 0.75%, 1%, 2% Irganox 1098, 0.2% PolyAd 1932, and no
antioxidant respectively. The mean diameter was .about.400 nm. The
nanowebs were bonded to samples of polyester felts of basis weight
14 oz/yd.sup.2 by adhesive lamination as follows.
A discontinuous layer of polyurethane adhesive was applied to one
surface of the felt using a gravure roll. The felt and the nanoweb
were fed into a nip of two rolls with the adhesive-coated surface
of the felt contacting the nanoweb. The roll temperature was
144.degree. C., the nip pressure was 40 pounds per square inch
(psi) and the line speed was 3 meters per minute. The composite was
rolled up and tested. The control sample with no antioxidant was
laminated on a commercial machine. The nip roll temperature was
290.degree. F. and the line speed was 3 meters per minute.
All the samples and the control were tested in an oven at
150.degree. C. for 70 hours and the color was noted at the end.
Table 3 shows the air permeability, basis weight, air permeability,
and pressure drop before heat aging. Table 4 shows filtration
efficiency and cycle time data before aging, and the color of the
nanoweb after aging. Table 5 below shows the filtration efficiency,
pressure drop and cycle time measured according to VDI 3926.
TABLE-US-00003 TABLE 3 AO Level and Total Basis Pressure Sample
type. Weight gsm Permeability Drop Pa 6 0.75% 1098 507 43.0 not
available 7 1.0% 1098 513 46.1 258 8 2.0% 1098 521 39.4 248 9 0.2%
PolyAd 505 43.3 330 10 0 483 69.2 268
TABLE-US-00004 TABLE 4 Efficiency Cycle Time Sample Mgm.sup.-2
(seconds) Color 6 Not available Not available White 7 20 253 White
8 20 296 White 9 30 207 White 10 20 260 Light Brown
The lamination process of webs with Irganox was acceptable and the
product shows improved high temperature durability with no
deterioration in other properties.
* * * * *